The present invention relates to improvement of a method of making a paper consisting essentially of a cellulose fiber or the said fiber and an inorganic filler, and in particular, to a papermaking method in which the retention of the cellulose fiber and the inorganic filler, especially fines thereof, in the sheet-formation step is extremely improved and the drainage in the said step is also elevated and additionally the paper strength of the dried sheet is improved.
More particularly, the present invention relates to improvement of a papermaking method, in which a cationic starch ether derivative which is characteristically in the form of a sulfamic acid salt itself is added to an aqueous papermaking stock containing a cellulose fiber or the said fiber and an inorganic filler, prior to the sheet-formation step of the method.
A paper which consists essentially of a cellulose fiber or the said fiber and an inorganic filler is, in general, prepared by a method comprising the step of feeding an aqueous papermaking stock obtained by dispersing and beating a cellulose pulp in water and adjusting the concentration of the resulting slurry and optionally adding a filler and any other additives thereto, to a sheet-formation machine, and the subsequent steps of dewatering and drying the resulting sheet. It is, however, an important fact that a pulp of high quality is lacking for the paper manufacturing, and the amounts of use of a pulp of low quality so as to compensate it and also of the inorganic filler are increasing for the purpose of lowering the manufacturing cost of paper. Hence the improvement of the retention and of the drainage in the sheet-formation step to enhance the productivity, the improvement of the product paper quality, etc. in the manufacturing of paper have been more and more desired.
In order to improve the retention in the sheet-formation step, a method of adding an aluminium sulfate as a coagulant to a papermaking stock has been carried out. In this method, however, the papermaking stock is required to be made acidic, prior to being subjected to the sheet-formation step. By reason of such matter, the durability of the final product paper would become lowered, the re-use of the white water would become difficult, the apparatus used would be corroded, and further the filler to be used is limitative as calcium carbonate cannot be used.
U.S. Pat. No. 2,876,217 has proposed to add a cationic starch in the form of a hydrochloric acid salt to a papermaking raw material dispersion. This is, however, still insufficient in the point of both the improvement of the retention in the sheet-formation step and the improvement of the strength of the paper product.
Japanese patent application (OPI) No. 6587/80 (the term "OPI" as used herein means a "published unexamined Japanese patent application") illustrates to add a combination comprising a cationic component such as a cationic starch, an acrylamide copolymer, etc. and an anionic component such as a natural starch, a carboxymethylated starch, etc., to a papermaking stock prior to the sheet-formation step. However, this is silent about the use of a cationic starch in the form of a sulfamic acid salt.
Japanese patent application (OPI) No. 12824/80 also illustrates a method of adding a combination of a cationic resin emulsion and an anionic water-soluble polymer, to a papermaking stock. However, the conventional cationic starch which has hitherto been utilized in the prior art is in the form of a hydrochloric acid salt. If this is used in the papermaking method as illustrated in Japanese patent application (OPI) No. 6587/80, the drainage in the sheet-formation step is insufficient and therefore the productivity is poor and additionally the paper quality of the final product is low. Even by the method as illustrated in Japanese patent application (OPI) No. 12824/80, the drainage and the paper quality of the final product are not satisfactory because of the same reason as above.
Japanese patent application (OPI) No. 51900/82 illustrates a papermaking method in which a combination of a cationic starch in the form of a hydrochloric acid salt and an anionic aqueous silicasol is added to an aqueous papermaking stock and the resulting dispersion is subjected to the sheet-formation step. According to the said method, the drainage and the paper quality of the final product are fairly good, but the retention in the sheet-formation step would still be insufficient in some cases depending upon the sheet-formation conditions. In particular, the retention of the inorganic filler would often be lowered, because of the addition of the cationic starch in the form of a hydrochloric acid salt.